Efficient and simple method for metalorganic chemical vapor deposition

ABSTRACT

The present invention provides a method of preparing metal or metal oxide particles on a substrate by forming a reaction mixture of a metal or metal oxide precursor and a substrate, and heating the reaction mixture at reduced pressure, such that metal or metal oxide particles are formed on the substrate.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/539,142, filed Sep. 26, 2011, which is incorporated in its entiretyherein for all purposes.

BACKGROUND OF THE INVENTION

Metalorganic chemical vapor deposition (MOCVD) is a process used tosynthesize materials by means of a chemical reaction in a vapor of ametal-organic precursor. The chemical reaction breaks apart theprecursor molecules and deposits metals or metals oxides on a substratematerial. The organic portion of the precursor returns to the gaseousphase. The deposited metals or metal oxides may be particles withnanometer sized dimensions (and therefore, large surface area to massratios)

It is often desirable that the MOCVD reactions occur on a specificsubstrate surface, such that the surface is covered by discreteparticles, a network of particles, or a continuous film of depositedmaterial. Previously reported MOCVD methods use a flowing stream of theprecursor vapor, which may be mixed with an inert carrier gas. Thereactant stream flows over or through the substrate. The substrate isoften heated, so as to induce the chemical conversion of molecules ofprecursor which collide with the substrate surface from the vapor phase.This heating may be applied to the entire deposition chamber (“hot-wall”CVD) or may be applied only to the substrate by means of inductiveheating (“cold wall” CVD). A catalytic “hot wire” or a plasma may alsobe present to encourage the deposition reaction.

Convection based MOCVD methods show poor performance in depositinguniform coatings on powders and highly porous materials/structures,which may have a very large surface areas and do not present allavailable surfaces to a convective reactant flow. Some methods devisedto cope with this problem for powdered materials are fluidized bedreactors that agitate the substrate powder by flowing the reactant vaporthrough the powder via a fritted disc. However, to date known methodsdevised to deposit metal or metal oxide particles on powders and highlyporous materials/structures are inefficient with respect to the use ofthe metal-organic precursors and/or complex and expensive in design,inhibiting the commercial viability of such methods. A method forefficiently depositing metal/metal oxide particles on powders and highlyporous materials/structures was therefore developed.

Metal or metal oxide nanoparticles supported on the surfaces of highsurface area materials/structures are required for the catalysis ofchemical reactions in many chemical and electrochemical devices. Forboth chemical and electrochemical devices, these functional metal ormetal oxide particles are dispersed on supporting materials. In the caseof chemical devices, the supporting material is usually a chemically andthermally stable material with high surface area. For electrochemicaldevices, the functional metal or metal oxide particles are typicallydispersed on electrically conductive supports or inside highly porouselectrodes which conduct both electrons and ions, and have high gaspermeability.

Chemical devices containing functional metal or metal oxide materialsperform a multitude of heterogeneous reactions. Such reactions includefuel reforming (e.g. steam reforming, partial-oxidation, auto-thermal),hydrolysis (e.g., hydrogen production from water), hydrogenation (e.g.,reaction of hydrogen with fats to form margarine), and more generically,catalytic reduction (e.g. ammonia formation from nitrogen) and oxidation(e.g., water gas shift and nitric acid formation from ammonia)reactions. Highly dispersed metal or metal oxide catalyst particles withhigh surface areas are desirable as increased catalyst surface areatypically increases overall device reaction rates.

Electrochemical devices with functional metal or metal oxide materialsin their electrodes are used in variety of applications includingbatteries, fuel cells, hydrogen pumps, electrolysis cells,supercapacitors, sensors, hydrogen separation membranes and membranereactors. Highly dispersed and high surface area metal or metal oxideparticles in the electrodes of electrochemical devices are oftenessential in promoting fast reaction and intercalation rates, and moregenerally, the fast charge transfer rates measured in highly activeelectrodes. The overall functionality of most electrochemical devices isthen highly dependent on the chemical and physical nature of thefunctional metal or metal oxide particles.

In particular, the chemical and physical nature of the functional metalor metal oxide particles in fuel cell electrodes are critical to highperformance and fuel conversion efficiencies, and therefore critical tothe overall commercial viability of a fuel cell system. Fuel cells areattractive alternatives to combustion engines for power generation,because of their higher efficiency and the lower level of pollutantsproduced from their operation. A fuel cell generates electricity fromthe electrochemical reaction of a fuel, e.g. methane, methanol,gasoline, or hydrogen, with oxygen normally obtained from air.

There are three common types of fuel cells i.e., 1) direct hydrogen/airfuel cells, in which hydrogen is stored and then delivered to the fuelcell as needed; 2) indirect hydrogen/air fuel cells, in which hydrogenis generated on site from a hydrocarbon fuel, cleaned of carbonmonoxide, and subsequently fed to the fuel cell; and 3) direct alcoholfuel cells, such as methanol (“DMFC”), ethanol, isopropanol and thelike, in which an alcohol/water solution is directly supplied to thefuel cell. An example of this later fuel cell was described, forexample, in U.S. Pat. No. 5,559,638, the disclosure of which isincorporated herein by reference.

Regardless of the fuel cell design chosen, the operating efficiency ofthe device is partly limited by the efficiency of the electrolyte attransporting ions (e.g., protons, oxygen vacancies, hydroxyl ions,bicarbonate ions, etc.). Typically, perfluorinated sulfonic acidpolymers, sulfonated hydrocarbon polymers, and composites thereof areused as electrolyte membrane materials for proton conducting fuel cells.However, these conventional materials utilize hydronium ions (H₃O⁺) tofacilitate proton conduction. Accordingly, these materials must behydrated, and a loss of water immediately results in degradation of theconductivity of the electrolyte and therefore the efficiency of the fuelcell.

As a result, fuel cells utilizing these materials require peripheralsystems to regulate water evaporation rates. If water is flushed fromthe system too quickly, the system will dry out, and the conductivity ofthe polymer electrolyte will decrease. If water is removed too slowly,liquid water can form and flood the porous volumes within theelectrodes, blocking the access of gaseous species. These peripheralsystems increase the complexity and cost of these fuel cells, from theuse of expensive noble catalysts (Pt) to temperature requirements thatcannot exceed much above 100° C. As a result of these temperaturelimitations, the fuel cell catalysts and other systems cannot beoperated to maximum efficiency. Higher temperatures can also reducecarbon monoxide poisoning of the fuel cell catalyst.

It has been shown that the solid acids such as CsHSO₄ can be used as theelectrolyte in fuel cells operated at temperatures of 140-160° C.(Haile, S. M., et al. Nature 2001, 410, 910-913). The high conductivityof CsHSO₄ and analogous materials results from a structural phasetransition (referred to as a superprotonic phase transition) that occursat 141° C. from an ordered structure, based on chains of SO₄ groupslinked by well-defined hydrogen bonds, to a disordered structure inwhich SO₄ groups freely reorient and easily pass protons between oneanother. Across this transition, the proton conductivity increases by 3to 4 orders of magnitude from 10⁻⁶ Ω⁻¹cm⁻¹ (phase II) to 10⁻³-10⁻²Ω⁻¹cm⁻¹ (phase I; Baranov, A. I., et al. JETP Lett. 1982, 36(11),459-462). Thus, disorder in the crystal structure is a key prerequisitefor high proton conductivity.

However, the lifetime of these sulfate and selenium based“superprotonic” solid acids is short (Merle, R. B., et al. Energy &Fuels 2003, 17, 210-215) when operated under standard fuel cellconditions. The short lifetime of both CsHSO₄ and CsHSeO₄ under fuelcell operating conditions results from the reduction of sulfur andselenium by hydrogen in the presence of typical fuel cell catalysts,according to:2CsHSO₄+4H₂→Cs₂SO₄+H₂S+4H₂O2CsHSeO₄+4H₂→Cs₂SeO₄+H₂Se+4H₂O

Accordingly, research was done to find a “superprotonic” solid acidelectrolyte stable under fuel cell conditions, resulting in thedemonstration that CsH₂PO₄ (CDP) has as superprotonic transition and isstable under fuel cell conditions (Boysen, D. A., et al. Science 2004,303, 68-70). For CsH₂PO₄, the superprotonic transition is at 231° C.,above which the material has a high proton conductivity: e.g., 2.5×10⁻²Ω⁻¹cm⁻¹ at 250° C. Solid acid fuel cells (SAFCs) using CsH₂PO₄ thenoperate at intermediate temperatures (˜230-280° C.), are inherentlyimpermeable to gases, and transport “bare” protons through a solidelectrolyte. These properties give SAFCs advantages over other fuel celltechnologies in cost, durability, start/stop cycling, fuel flexibility,and simplified system design. To date, solid acid fuel cells (SAFCs)utilizing this electrolyte as thin (10-25 μm) gas tight electrolytelayers have demonstrated peak power densities of over 330 mW/cm² onhydrogen/air with lifetimes of thousands of hours. Moreover, SAFC stackshave demonstrated robustness to thermal cycling, power outputs of over 1kW, and degradation rates similar to those of single cells suggestingstack lifetimes in the thousands of hours.

SAFCs have also demonstrated very high tolerances to typical anodecatalyst poisons such as carbon monoxide (CO), ammonia (NH₃), andhydrogen sulfide (H₂S): measured tolerances are 20%, 100 ppm, and 100ppm, respectively, without significant performance decreases. These highimpurity tolerances and the intermediate operating temperatures allowSAFC power systems to operate on reformed fuels with simplified systems.Taken together, the advantageous properties of SAFCs are anticipated toresult in relative low SAFC stack and system costs, specifically becauseof: 1) easy cell and stack fabrication, 2) durable on/off cycling, 3)standard metal and polymer stack components, and 4) simplified systems.SAFCs thus provide the vast majority of the benefits of both high andlow temperature systems, but few of their disadvantages. That is, SAFCsoperate at high enough temperatures to run effectively on a wide rangeof reformed fuels, but not so hot that the thermal stability/cost ofstack and system components limits commercial viability.

To achieve the high performance and stability of current SAFCs, it wasnecessary to increase the interaction between the catalyst particles(typically, platinum) used in SAFC electrodes and the solid electrolytewhile maintaining reactant gas access, as catalytic reactions only takeplace on catalyst particles both in contact with the electrolyte andreactant gases. This motivated the development of a deposition method toplace the catalyst particles directly on the surface of the solid acidelectrolyte. As the electrolyte is water soluble, most common aqueousmethods for creating and depositing catalyst particles on substratematerials would not be suitable for use with solid acid electrolytes.Therefore, the simple method described herein for depositing catalystparticles from the gas phase was developed.

This method is applicable to a broad range of chemical andelectrochemical devices due to the simple and effective manner in whichmetal and metal oxide particles can be deposited onto the surfaces ofelectrodes, high surface area powders, and other porous materialstypically used in such devices.

BRIEF SUMMARY OF THE INVENTION

The invention provides a method of preparing metal or metal oxideparticles on a substrate. The invention includes forming a reactionmixture of a metal or metal oxide precursor and a substrate; and heatingthe reaction mixture at reduced pressure, such that metal or metal oxideparticles are formed on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows metalorganic vapor deposition method in the case ofdeposition on a powder substrate. (1) is a container holding thesubstrate and one or more metalorganic precursors (2) a small volume ofliquid water intended to vaporize during the deposition process (3) Asealed vessel containing a heating element (4) valves allowing forevacuation of the vessel (3), closed during deposition. (5) Vacuum pump(6) Inert purge gas

FIG. 2 shows deposition of single-element nanoparticles (3) on powdersubstrate particles (2) following vaporization and decomposition ofsolid precursor particles (1).

FIG. 3 shows deposition of A-B alloy nanoparticles (4) on powdersubstrate particles (3) following vaporization and decomposition ofsolid precursor particles of element A (1) and element B (2).

FIG. 4 shows scanning electron micrographs of Pt nanoparticles formedwithin the pores of a porous disc of CsH₂PO₄ after processing asdescribed in Example 2.

FIG. 5 shows scanning electron micrograph of Pt—Pd alloy deposited onCsH₂PO₄ via method similar to that described in Example 3.

FIG. 6 shows Pt 4f and Pd3d XPS spectra for Pd₈₀Pt₂₀ nanoparticlesformed on the surface of CsH₂PO₄ particles after processing similar tothat described in Example 3. Samples were washed in water to removeCsH₂PO₄ particles and leave only Pt—Pd nanoparticles.

FIG. 7 shows a scanning electron micrograph (top) and associated EDSspectrum (bottom) of Pt—Pd alloy deposited on CsH₂PO₄ via a methodsimilar to that described in Example 3.

FIG. 8 shows a single-particle EDS spectrum for Pd—Pt nanoparticlesformed on a copper TEM grid (inset).

FIG. 9 shows the Cu Kα x-ray diffraction patterns of Pt nanoparticlesdeposited on nanoscale silicon carbide and a scanning electronmicrograph of the sample (inset).

FIG. 10 shows scanning electron micrographs of Pt nanoparticlesdeposited on CsH₂PO₄. (a) 5 wt % Pt (b) 9 wt % Pt (c) 17 wt % Pt (d) 29wt % Pt.

FIG. 11 shows Cu Kα x-ray diffraction patterns of Pt nanoparticles (a)deposited on CsH₂PO₄ and (b) with the water-soluble CsH₂PO₄ substrateremoved by dissolution.

FIG. 12 shows deposition of continuous nanoparticle films (3) within aporous substrate material (1). A powder of precursor material (2) isplaced in contact with the substrate prior to deposition heat treatment;a nanoparticle film is formed within the pores of the substrate material

FIG. 13 shows a scanning electron micrograph of Pt nanoparticle filmformed within the pores of an anodic aluminum oxide porous membranematerial.

FIG. 14 shows temperature and pressure profiles of a container duringdeposition of Pt on CsH₂PO₄ from Pt(acac)₂.

FIG. 15 shows an SEM micrograph of a CDP-Pt(acac)₂ precursor mixturecontaining 29 wt % Pt(acac)₂ prior to platinum deposition.

FIG. 16 shows XPS scans of the Pt 4d doublet of Pt:CDP samples. Solidlines are fits to Pt⁰ peaks, and dashed lines are fits to Ptoxide/hydroxide peaks. A small C 1s loss peak at 313 eV is omitted inthe displayed peaks.

FIG. 17 shows XPS scans of the O 1s peak of Pt:CDP samples. Theassignment of peaks 1, 2, and 3 is described in the text.

FIG. 18 shows XPS scans of the C 1s peak of Pt:CDP samples. The larger,adventitious carbon peak at 284.8 eV was used as an internal energystandard, and is fit with a solid line. The dashed peak is assigned toorganic species resulting from retained organic fragments of thePt(acac)₂ precursor.

FIG. 19 shows XPS scans of the P 2p peak of Pt:CDP samples and thesingle fitted peak.

FIG. 20 shows SEM micrograph of 400 mg of Pt (50 wt %, 50 nm) depositedon 200 mg of CDP powder.

FIG. 21 shows plot of (a) voltage drop from 1.1 V at 50 mA/cm² and (b)ohmic resistance of SAFC electrodes, as a function of platinum loading.Each electrode contained 50 mg of CDP.

FIG. 22 shows IR-free polarization curves of a standard SAFC electrodeand a Pt:CDP electrode.

FIG. 23 shows cross-section electron micrographs of SAFC electrodes: (a)standard SAFC electrode containing Pt black and (b) Pt:CDP electrode.

FIG. 24 shows an electron micrograph of a Pt:CDP cathode after fuel celloperation. The bright Pt nanoparticles have reorganized on the darkerCDP surface.

DETAILED DESCRIPTION OF THE INVENTION I. General

The present invention provides a method for depositing a metal, a metalalloy, or metal oxide particles on a substrate material using ametal-organic precursor material in the absence of convective flow.Advantages of the method include: a surprisingly uniform coating ofsubstrate materials; an extremely high yield of deposited particles withrespect to the precursor metal; a simple and inexpensive apparatus; andlow process temperatures in comparison with existing methods. Ametalorganic precursor of the material to be deposited is placed inclose contact with the substrate and heated in a deposition chamber. Thegases in the deposition chamber have a low oxygen partial pressure, andthe method can be conducted with a quantity of liquid water whichvaporizes during the heating process.

II. Definitions

“Metal or metal oxide particles” refers to particles with averageparticle size (APS) or diameter in the range of 1-50 nm that aredeposited from the vapor phase by the decomposition of precursors on thesurface of the substrate material. Particles having a high surface areaper unit mass can increase overall chemical or electrochemical reactionrates when employed in a device such as a fuel cell. Such particles canhave APSs in the range of 1-10 nm and, preferably, in the range of 1-5nm.

“Metal or metal oxide precursor” refers to a chemical compound having ametal atom bound by one or more coordinating organic ligands that candissociate from the metal atom to produce particles of that metal or anoxide of that metal, and form the metal or metal oxide particles of thepresent invention.

“Metal” refers to elements of the periodic table that are metallic andthat can be neutral, or negatively or positively charged as a result ofhaving more or fewer electrons in the valence shell than is present forthe neutral metallic element. Metals useful in the present inventioninclude the alkali metals, alkali earth metals, transition metals andpost-transition metals. Alkali metals include Li, Na, K, Rb and Cs.Alkaline earth metals include Be, Mg, Ca, Sr and Ba. Transition metalsinclude Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru,Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg and Ac.Post-transition metals include Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi, andPo. One of skill in the art will appreciate that the metals describedabove can each adopt several different oxidation states, all of whichare useful in the present invention. In some instances, the most stableoxidation state is formed, but other oxidation states are useful in thepresent invention.

“Metal oxide” refers to the oxide of any metal such as alkaline earthmetals, transition metals, as well as post-transition metals. One ofskill in the art will appreciate that other metal oxides are useful inthe present invention.

“Metal salt” refers to ionic compounds formed by reaction of a metalwith an acid or a base such that an ionic bond is formed between themetal ion and the counterion. Any suitable metal can be used to form themetal salt, as well as any suitable acid or base.

“Substrate” refers to a material upon which the metal or metal oxideparticles are formed. Any suitable substrate can be used, including, butnot limited to, a metal, metal oxide, metal salt, carbon material,silica material, polymer, solid acid or solid oxide.

“Carbon material” refers to any solid material containing allotropicforms of carbon, including crystalline and amorphous phases, andmixtures of these phases in composites or solid solutions with othersolid materials.

“Silica material” refers to any solid material containing allotropicforms of silicon dioxide (SiO₂), including crystalline and amorphousphases, and mixtures of these phases in composites or solid solutionswith other solid materials.

“Polymeric material” refers to a high-weight, macromolecular solidmaterial components characterized by at least twenty, and preferablymore, repeating low-molecular-weight monomer units.

“Solid acid” refers to compounds with the chemical formulaM_(a)H_(b)(XO_(c))_(d).nH₂O, wherein M is a metal cation having chargefrom +1 to +2, and X is one or more species selected from Si, P, S, As,Se, Te, Cr, and Mn. Subscripts a, b, c, d, and n are rational numbers.In some embodiments, M includes one or more species selected from Li,Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Ti, and NH₄.

“Solid oxide” refers to materials suitable in a solid oxide fuel cell,such as yttria stabilized zirconia (YSZ), scandia stabilized zirconia(ScSZ) and gadolinium doped ceria (GDC). Other solid oxides are usefulin the methods of the present invention.

The substrate can also be in any suitable form, such as a solid, aporous solid or a powder. “Powder” refers to a finely divided solidmaterial with average particle sizes ranging from tens of nanometers tohundreds of microns. “Porous solid” refers to any solid materialcomposed of a nonzero volume fraction of void space. “Solid substrate”refers to a solid material that is substantially free of void space.

“Forming a reaction mixture” refers to the process of bringing intocontact at least two distinct species such that they mix together andcan react, either modifying one of the initial reactants or forming athird, distinct, species, a product. It should be appreciated, however,the resulting reaction product can be produced directly from a reactionbetween the added reagents or from an intermediate from one or more ofthe added reagents which can be produced in the reaction mixture.

“Reduced pressure” refers to a pressure less than atmospheric pressure.

“Water soluble” refers to the characteristic ability of a compound orcomposition to dissolve in water.

“Continuous network” refers to a structure of arbitrary fractaldimension, composed of adjacent sites occupied by active elements suchthat an unbroken percolation path defined by active element sites existsbetween the boundaries of the structure. The continuous network can be acontinuous, electrically conductive network defined by an active elementwith the property of electrical conductivity.

“Solvent” refers to water-miscible or -immiscible solvents capable ofdissolving either or both of water-soluble and water-insolublecompounds. Exemplary solvents useful in the present invention include,but are not limited to, alcohols, acids, polyols and otherwater-miscible organic solvents such as propylene carbonate, N-methylpyrrolidone, di-methyl sulfoxide and di-methyl formamide. Other usefulsolvents include halogenated solvents such as dichloromethane,chloroform and carbon tetrachloride. One of skill in the art willappreciate that other organic solvents are useful in the presentinvention.

III. Preparation of Metal or Metal Oxide Particles on a Substrate

The present invention provides methods for producing metal, metal alloy,or metal oxide particles from metal-organic precursor materials in theabsence of convective flow. Particles are formed from precursors in thevapor phase on a substrate material in high yield, affordinguniformly-coated substrates using low process temperatures.

Accordingly, the invention provides a method of preparing metal or metaloxide particles on a substrate. The method includes forming a reactionmixture of a metal or metal oxide precursor and a substrate, and heatingthe reaction mixture at reduced pressure, such that metal or metal oxideparticles are formed on the substrate.

Precursors

In general, the metal or metal oxide precursors used in the methods ofthe invention are metal-organic species containing a metal atom bound byone or more coordinating ligands. In some embodiments, the metal ormetal oxide precursor includes at least one metal selected from atransition metal and a post-transition metal. In some embodiments, themetal or metal oxide precursor includes at least one metal selected fromPt, Pd, Ir, Ru, Ni, Co, Fe, Cu, V, Cr, Ti, Ta, Mn, Mo, Nb, and Sn. Insome embodiments, the metal or metal oxide precursor includes at leastone metal selected from Pt, Pd and Ru. In some embodiments, the metal ormetal oxide precursor includes Pt. In some embodiments, the metal ormetal oxide precursor includes Pt and Pd. One of skill in the art willappreciate that other combinations of metals can be useful as metal ormetal oxide precursors in the methods of the invention.

More than one precursor can be used simultaneously in the method toproduce nanoparticles or a nanoparticle film or network that is composedof more than one metal, alloy, intermetallic compound, or metal oxide.This can be accomplished by addition of both precursors into directcontact with the substrate prior to heat treatment in low oxygen partialpressure. In the case of mixed precursors, both precursors can vaporizeand deposit atoms, clusters, or particles concurrently on the substrate,forming solid solution nanoparticles or multiphase particles dependingon the relative volatility of the precursors and the intrinsic mixingthermodynamics of the deposited species. Metals for which this approachis applicable include Pt, Pd, Ir, Ru, Rh, Ni, Co, Fe, Cu, V, Cr, Ti, Ta,Mn, Mo, Nb, and Sn, among others.

Any suitable metal-organic precursor can be used in the methods of theinvention. Suitable ligands for the metal-organic precursor include, butare not limited to, acetylacetonate and fluoroacetylacetonates such astrifluoroacetylacetonate and hexafluoroacetylacetonate. In someembodiments, the precursor includes platinum (II) acetylacetonate. Insome embodiments, the precursor includes platinum (II) acetylacetonateand palladium (II) acetylacetonate.

Substrates

The precursor deposition process of the invention is broadly applicableto a wide range of substrates. In some embodiments, the substrateincludes at least one member selected from a metal, a carbon material, ametal oxide, a silica material, a polymeric material, a solid acid, asolid oxide, and a metal salt.

Nearly all oxides collect water, hydroxyl groups, or both, and metalstypically exhibit passive oxide films that are similarly hydroxylated.Without wishing to be bound by any particular theory, it is believedthat hydroxyl groups on a substrate surface displace organic ligandsfrom a metal-organic precursor and promote the particle formationprocess. For this reason, materials with surface hydroxylgroups—including metals and metal oxides, as well as metal salts, carbonmaterials, silica materials, and polymers—can be useful substrates inthe present invention. In some embodiments, the substrate is selectedfrom Fe and its alloys, including stainless steel. In some embodiments,the substrate is selected from Ni, Al, Sn, Zn, Ag, Ta, Ru, Zr, Ti, Co,Cu, Zn, Mn, Au, Pd, and Pt, or from alloys oxides thereof.

Metals useful as substrates in the methods of the invention include Al,Cd, Ca, Ce, Cr, Co, Cu, Gd, Ga, Au, In, Ir, La, Pb, Li, Mg, Mn, Mo, Nd,Ni, Pd, Pt, Rh, Ru, Sm, Ag, Sr, Ta, Sn, Ti, V, Y, Zn, and Zr. In someembodiments, the metal substrate can be Al, Ce, Cr, Co, Cu, Au, In, Ir,Pb, Li, Mg, Mn, Mo, Nd, Ni, Pd, Pt, Rh, Ru, Ag, Sr, Ta, Sn, Ti, V, Y,Zn, or Zr. In other embodiments, the metal substrate can be Pt, Pd, Ir,Ru, Rh, Ni, Co, Fe, Cu, V, Cr, Ti, Ta, Mn, Mo, Nb, or Sn. Metal oxidesuseful as substrates in the methods of the invention include oxides ofAl, Cd, Ca, Ce, Cr, Co, Cu, Gd, Ga, Au, In, Ir, La, Pb, Li, Mg, Mn, Mo,Nd, Ni, Rh, Ru, Sm, Ag, Sr, Ta, Sn, Ti, V, Y, Zn, and Zr.

In some embodiments, the substrate can be a pure or mixed oxide ofsilicon. Silicon can be present in various allotropic forms, includingquartz, fused quartz, other silicate glasses, fumed silica, andmesoporous silicas such as MCM-41. In some embodiments, the substratecan be titanium dioxide, in the anatase phase, rutile phase, or mixturesthereof. Titanium dioxide can be doped with other elements such asnitrogen to enhance electronic conductivity and catalytic interactionswith the deposited metal or metal oxide particles. In some embodiments,the substrate can be pure or mixed oxides of aluminum, such as sapphire,nanoparticulate aluminum oxide, or porous monoliths of anodic aluminumoxide.

In some embodiments, the substrate can be carbon in various allotropicforms such as diamond, graphite, graphene, graphene oxide, nanotubes,Bucky balls, and turbostratically disordered carbon blacks (includingacetylene black and Vulcan XC-72). In some embodiments, the substratecan be a polymer, such as polybenzimidazole and other imidazole-basedpolymers, or perfluorinated sulfonic acid polymers such as Nafion.

In some embodiments, the substrate includes a solid acid. Examples ofsolid acids include, but are not limited to, Cs₅(HSO₄)₃(H₂PO₄)₂,Cs₃(HSO₄)₂(H_(1.5)(S_(0.5)P_(0.5))O₄), Cs₅H₃(SO₄)₄.xH₂O, TlHSO₄,CsH(SeO₄)₄, Cs₂(HSeO₄)(H₂PO₄), (NH₄)₃H(SO₄)₂, (NH₄)₂(HSO₄)(H₂PO₄),Rb₃H(SO₄)₂, Rb₃H(SeO4)₂, Cs_(1.5)Li_(1.5)H(SO₄)₂, Cs₂Na(HSO₄)₃,TlH₃(SeO₃)₂, CsH₂AsO₄, (NH₄)₂(HSO₄)(H₂AsO₄), CaNaHSiO₄,Cs₂(HSO₄)(H₂PO₄), CsHSO₄ and CsH₂PO₄. Hydrates of solid acids,containing varying amounts of water, can also be used as substrates inthe present invention. CsH₂PO₄, in particular, can be used a substratein various forms. Given a solid CsH₂PO₄ monolith as the substrate, aprecursor can be applied as a solid powder, slurry or suspension, orfrom solution onto the membrane and subjected to the appropriatetemperature and partial pressure conditions to deposit nanoparticles ora nanoparticle film on the surface of the CsH₂PO₄ monolith for use as aplanar electrode in a fuel cell or similar device. The process can beperformed when CsH₂PO₄ is mixed with other materials, such as metals,oxides, carbides, polymers, carbon materials, and silica materials, toform dense composites.

The methods of the invention are also applicable to the deposition ofnanoparticles or a nanoparticle film or network on fine powders ofCsH₂PO₄ with APS from 10 nm to 100 micron. Similarly it can be appliedto a powdered mixture of CsH₂PO₄ and other solid materials as describedabove, resulting in deposition of nanoparticles or a nanoparticle filmor network on both the CsH₂PO₄ and the added solid materialssimultaneously.

The method is also applicable to the deposition of nanoparticles or ananoparticle film or network in a porous solid composed of CsH₂PO₄.Porous CsH₂PO₄ can be fabricated by pressing powders with a particularAPS (generally 10 nm-100 μm) in a uniaxial die at a suitably lowpressures. At sufficiently low pressures, the compact does not becomefully dense but rather retains an open-cell porosity with pore sizes onthe order of the starting powder APS. Pore density can be controlled bythe pressure applied in the die. Nanoparticles or a nanoparticle film ornetwork can then be deposited within the porous CsH₂PO₄ solid. Poroussolids composed of CsH₂PO₄ and additional components, such as metals,oxides, carbides, polymers, carbon materials, and silica materials, canalso be fabricated in this manner.

Any suitable combination of precursor and substrate can be used in themethods of the invention. In some embodiments, the substrate includes atleast one of CsH₂PO₄ and alumina. In some embodiments, the substrateincludes CsH₂PO₄. In some embodiments, the metal or metal oxideprecursor includes Pt, and the substrate includes CsH₂PO₄.

The substrate can be in any suitable physical form. For example, thesubstrate can be a solid block with a minimum of porosity.Alternatively, the substrate can be a solid with a substantial amount ofporosity. Moreover, the substrate can be a powder. In some embodiments,the substrate can be a powder, a porous substrate or a solid substrate.In some embodiments, the substrate can be a porous substrate or a solidsubstrate, such that the metal or metal oxide nanoparticles form acontinuous network of particles on the surface of the substrate. In someembodiments, the precursor can be platinum (II) acetylacetonate and thesubstrate can be CsH₂PO₄. In some embodiments, the metal precursorincludes platinum (II) acetylacetonate and palladium (II)acetylacetonate, and the substrate can be CsH₂PO₄. One of skill in theart will appreciate that other combinations of precursors and substratescan be useful in the methods of the invention.

Processing Conditions

The methods of the invention afford particle-coated substrates usingadvantageous processing conditions, including relatively lowtemperatures and moderate pressures, where both the metal or metal oxideprecursor and the substrate are in the same reaction vessel. In someembodiments, the methods of the invention include contacting a metal ormetal oxide precursor and a substrate at low oxygen partial pressure;heating the metal or metal oxide precursor under conditions sufficientto vaporize the precursor; and collecting the gaseous metal or metaloxide precursor on the substrate under conditions sufficient to formmetal or metal oxide particles, thereby depositing the particles on thesubstrate.

In the methods of the invention, metal or metal oxide precursors andsubstrate materials can be placed in direct physical contact in asuitable container. In some embodiments, the metal or metal oxideprecursor and the substrate are in direct physical contact. Containersuseful in the invention include evacuable vessels made of glass, metal,metal oxide, polymer, or combinations of these materials. In someembodiments, the container can be a glass, metal, metal oxide, orpolymeric cylinder or prism capable of being evacuated by a vacuum pump.In still other embodiments, the container is an insulated vacuum ovensuch as the Fisher Scientific Isotemp 281A Vacuum Oven.

Precursors can be applied to the substrate in various forms. Someembodiments of the invention provide methods as described above, whereinthe reaction mixture contains a precursor and a substrate withoutsolvent. Alternatively, the precursor or precursors can be applied to asubstrate as a slurry, suspension, or solution in a suitable solvent.Accordingly, some embodiments of the invention provide methods whereinthe reaction mixture includes a solvent selected from dichloromethane,chloroform and carbon tetrachloride. One of skill in the art willappreciate that other solvents can be useful in the methods of theinvention, depending on the nature of a particular precursor orsubstrate.

In general, the methods of the invention are conducted using atemperature program that: (a) heats from room temperature to theultimate temperature; (b) reaches a value high enough to cause thevaporization of the metal or metal oxide precursor; and (c) is lowenough that direct thermolysis of the metal or metal oxide precursor isnot the dominant decomposition mode. Some embodiments of the inventionprovide a method of preparing metal or metal oxide particles on asubstrate as described above, wherein heating the reaction mixture isconducted at a temperature of from about 20 to about 500° C. The heatingcan be a temperature of, for example, from about 20 to about 500° C., orfrom about 20 to about 450° C., or from about 20 to about 300° C., orfrom about 20 to about 250° C. The heating can be to a temperature ofabout 100° C., 110° C., 120° C., 130° C., 140° C., 150° C., 160° C.,170° C., 180° C., 190° C., 200° C., 210° C., 220° C., 230° C., 240° C.,or about 250° C. In some embodiments, the heating is at a temperature offrom about 20 to about 300° C. In some embodiments, the heating is at atemperature of from about 20 to about 210° C. One of skill in the artwill appreciate that the heating temperature will vary, depending inpart on the precursor or precursors employed in the method. ForPt(acac)₂, for example, the final temperature in the heating program isgenerally greater than 120° C. and lower than 300° C. The temperatureand pressure profile of the process used to produce Pt metalnanoparticles on CsH₂PO₄ from Pt(acac)2 precursor is shown in FIG. 14.

The method functions in a temperature regime that is above thevolatilization temperature of the precursors, but below the thermolysistemperature. As such, ligand dissociation from the metal-organicprecursors to produce the desire particles must be accomplished by aroute other than thermal decomposition. Without wishing to be bound byany particular theory, it is believed that precursor decompositionoccurs in the methods of the invention via interaction of surface waterspecies such as H₃O⁺, H₂O₂, H₂O, and OH⁻ with the precursor ligands.This is believed to result in chemical reactions that deposit thedesired material on the surface. Accordingly, the methods of theinvention can include heating the reaction mixture in an atmospherecontaining water vapor (or other gas-phase solvents) that can modulateparticle properties including, but not limited to, particlemicrostructure. In some embodiments, the heating is performed in thepresence of water vapor.

Any suitable pressure can be used in the methods of the invention. Ingeneral, heating of the reaction mixture is conducted at a pressure offrom about 0.01 to about 1 atm. The heating can be performed at apressure of, for example, from about 0.01 to about 1 atm, or from about0.02 to about 0.9 atm, or from about 0.25 to about 0.75 atm. The heatingcan be performed at a pressure of about 0.01 atm, 0.05, 0.10, 0.15,0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75,0.80, 0.85, 0.90, 0.95, or 1 atm. Some embodiments of the presentinvention provide methods as described above, wherein one or moreprecursor and one or more substrate materials are placed in directphysical contact in a sealed container at lower than atmosphericpressure and low oxygen partial pressures. In some embodiments, theheating is performed at a pressure of from about 0.01 to about 1 atm. Insome embodiments, the heating is performed at a pressure of from about0.2 to about 0.9 atm. In some embodiments, the heating is performed at apressure of from about 0.1 bar to about 0.9 bar.

“Low oxygen partial pressure” refers to a condition in a gas mixturesuch that the ratio of the number of moles of oxygen to the number oftotal moles of gas is lower than 0.1%. Oxygen partial pressures usefulin the invention are from about 0 bar to about 0.03 bar. The oxygenpartial pressure can be, for example, from about 0.01 bar to about 0.03bar, or from about 0.015 bar to about 0.02 bar. In some embodiments, theoxygen partial pressure can be about 0 bar, or about 0.005 bar, or about0.01 bar, or about 0.015 bar, or about 0.02 bar, or about 0.025 bar, orabout 0.03 bar.

Using the conditions described above, the present invention providesplatinum nanoparticles on CsH₂PO₄ (CDP) powder, platinum on a porousdisc of CDP, a mixture of platinum and palladium nanoparticles on CDPpowder, platinum nanoparticles on alumina, and ruthenium oxide on carbonblack.

Use of Particle-Coated Substrates

Substrates modified with nanoparticles, nanoparticle networks, ornanoparticle films can be used in electrochemical devices such asmembrane reactors, batteries, supercapacitors, electrolyzers, andhydrogen pumps. Hydrous ruthenium oxide supported on high surface areacarbon black, for example, can be used as the active material in anelectrochemical supercapacitor. The decorated particles can be formedinto a composite electrode with various active and inactive materials,as in the case of a polymer electrolyte membrane electrolyzer, whereiridium oxide supported on carbon black is mixed in an ink with aproton-conducting polymer suspension, and painted or sprayed on afibrous carbon gas diffusion medium for use as an electrode.

Particle-modified substrates can also be used in chemical devices suchas fuel reformers or catalytic reactors. For example, cobalt supportedon various oxides is useful for Fischer-Tropsch syntheses. Palladiumsupported on oxides such as SiO₂, Al₂O₃, or TiO₂ can be used for COoxidation devices. Palladium supported on ZnO can be used as a steamreforming catalyst in a reforming device used to produce hydrogen viacatalytic decomposition of hydrocarbons. Particle-modified substratescan also be used in photochemical devices such as photo-assistedchemical reactors or photoelectric solar cells. Platinum nanoparticlessupported on ZnO or TiO₂, for example, can be used for charge separationin a solar cell.

Substrates modified with nanoparticles, nanoparticle networks, ornanoparticle films can be used in fuel cells. Pt supported on variousforms of carbon, including graphite, graphene, and carbon black can beformed into a composite electrode with various active and inactivematerials, as in the case of a polymer electrolyte membrane fuel cell,in which the decorated particles are mixed in an ink with aproton-conducting polymer suspension, and painted or sprayed on afibrous carbon gas diffusion medium for use as an electrode. In anotheruse of the method, electrocatalyst particles can be deposited directlyon the woven or non-woven fibrous gas diffusion medium for use in a fuelcell.

The methods of the invention afford materials that can be used in any ofthe applications described above. The materials can be used forelectrochemical reactions involving one or more electrons as a reactant.In electrochemical devices and reactors, electron transfers happen at asurface and require connection to an external or internal circuit totransport electrons to and from the reaction site. Therefore, suitablematerials for electrochemical reactions are generally characterized byelectrical conductivity and a high surface area. The methods of thepresent invention can be used to deposit dispersed, high surface areacatalysts for electrochemical reactions (“electrocatalysts”) onelectrically conductive substrates such as nano-sized platinum dispersedon carbon black. Electrically conductive substrates such as metals,carbon, metal carbides, that act as supports for metallic, alloy, andoxide nanoparticles of metals such as Pt, Pd, Co, Ir, Rh, Ru, and Ni canbe used for electrochemical reactions converting one chemical species toanother.

The method can also be used to synthesize catalysts for chemicalreactions that do not explicitly require electrons as reactants. In thiscase, electrical conductivity is not necessarily required, but a highsurface area remains particularly useful. Substrates such asnanoparticles of aluminum oxide, silicon oxide, or titanium oxide,supporting metallic, alloy, and oxide nanoparticles of metals such asPt, Pd, Co, Ir, Rh, Ru, and Ni can be used for chemical reactionsconverting one chemical species to another.

These materials can be synthesized by mechanical mixing or agitation ofthe substrate (in particle or fiber form) with the appropriate precursoror precursors in a vessel made of glass, polymer, metal or composite ofsuitable size to bring all the added components into close contact, asin a compact or pile. In the case of a monolithic porous substrate, awoven fibrous substrate, or any other substrate with a definedmacroscopic structure that is to be maintained, the precursor orprecursor can be formed in a compact in contact with the substrate.After this compact is formed, deposition of the metal, alloy, or oxideparticles proceeds at low oxygen partial pressure and elevatedtemperature as specified.

As described above, it can be desirable for the nanoparticles depositedon a substrate to form a continuous network (i.e., a particle structureof arbitrary fractal dimension, composed of adjacent particles) or film(i.e., a two-dimensional or quasi-two-dimensional layer). In suchmaterials, electrons or ions can be conducted through the film ornetwork in an unbroken path from one boundary of the substrate toanother, or from one boundary of a composite material containing thesubstrate to another boundary. Examples of such materials includeplatinum deposited in a continuous nanoparticle film on CsH₂PO₄ used insolid acid fuel cells. CsH₂PO₄ is not electrically conductive, but acontinuous film of Pt can be formed on the surface of CsH₂PO₄ (see FIG.4) to act as both an electro catalyst and an electronic conductor,allowing an unbroken electrical circuit to be formed. The materials canalso be used in a battery intercalation reaction, in which an ion isstored in the crystal lattice of a host material, or in anelectrochemical capacitor, where charge is stored by changing theoxidation state of a metal or oxide particle.

When synthesizing materials of this type, it is useful to know thesurface area of the substrate in order to effectively form a continuousfilm or network on the substrate surface. Properly scaling the amount ofprecursor used in the method to the surface area of the substrate canhelp to control the size of the resulting particles and form apercolating network or film. In the case of Pt supported on CsH₂PO₄ asshown in FIG. 10, for example, 5 wt % Pt is not sufficient to produce apercolating electronic network on CDP particles with average particlesize around one micron (or surface area around 2.5 m²/g).

Another use of continuous networks or films is the synthesis ofnanostructures based on porous templates. A metal, alloy or oxide filmcan be coated on the internal structure of a porous material and thematerial removed via means such as dissolution, liberating a structurecomposed entirely of the deposited material with a shape dictated by theporous template. Yet another use of continuous networks or films are asprotective coatings, such as against chemical attack, or frictionalwear.

The methods of the invention can be used to prepare materials for solidacid fuel cells, as described above. The functioning of a fuel celldevice is determined in part by the structure of its electrodes. A fuelcell electrolyte is a solid capable of conducting ions between conjugatefuel cell electrodes or within a fuel cell electrode during the processof a fuel cell reaction or half-reaction, allowing chemical interactionsbetween the electrodes or within a single electrode. Fuel cellelectrodes can be formed from materials including catalyst supports,electrolytes, polymers, gas diffusion media, and current collectors.

The electrochemical reactions that define a fuel cell occur at so-calledtri-phase or triple-phase boundaries, which are zones characterized byan interface of a catalyst, electrolyte, and gas such that all phasesform continuous networks. The methods of the invention can be used todeposit nanoparticle networks on or within these electrolyte phases inservice of creating such networks in a fuel cell. This approach isapplicable to solid oxide materials such as cerium oxide, lanthanumstrontium manganite, lanthanum strontium cobalt ferrite, andyttria-stabilized zirconia. In other embodiments, this method can beapplied to solid electrolytes (i.e., an ion conductor that does notcontain a liquid phase) such as sulfated zirconia, or anhydrous doped orundoped tin pyrophosphates. In still other embodiments this method canbe applied to polymer electrolytes, such as alkaline exchange polymers,sulfonated hydrocarbons or perfluorinated sulfonic acid polymers such asAquivion, the 3M ionomer, or Nafion.

IV. Examples Characterization

Samples of each electrode powder were analyzed by scanning electronmicroscopy (SEM), energy dispersive spectrometry (EDS), x-rayphotoelectron spectroscopy (XPS), and x-ray diffraction (XRD).

SEM and EDS analyses were performed with a LEO 1525 FESEM system. Imageswere acquired at an accelerating voltage of 3 kV with the in-lenselectron detector, and EDS spectra were acquired at 15 kV with an OxfordINCA detector. Separate CDP-precursor mixtures were also preparedaccording to the procedure above, and micrographs were recorded at 10 kVto characterize the mixing and contact of CDP and Pt(acac)₂.

XPS experiments were carried out in an Omicron ultra high vacuum (UHV)chamber with a base pressure better than 5×10⁻¹¹ mbar. The chamber wasequipped with a PHI Al Kα X-ray source (10-610E) and monochromator(10-420), and an Omicron electron energy analyzer (EA-125). The sampleswere affixed to Si crystals (Ted Pella) with a uniaxial pressure of 100MPa. Before the measurement, the samples were outgassed at approximately100° C. using an indirect heater located underneath the sample holder.In addition to survey scans spanning a wide binding energy range with0.5 eV step size, high-resolution spectra were recorded with smallerstep size for the Pt 4d (0.2 eV), P 2p (0.1 eV), C 1s (0.2 eV), and O 1s(0.1 eV) photoelectron peaks. Adventitious carbon was used to calibratethe binding energy shifts of the sample (C 1s=284.8 eV). [Moulder, F.;Stickle, W.; Solbol, P.; Bomben, K. Handbook of X-ray PhotoelectronSpectroscopy; Perkin-Elmer Corp., 1992] Peak fitting was performed withthe XPSPeak [) Kwok, R. W. M. XPSPeak, 1999] software.

XRD was conducted on the as-synthesized samples with a Philips X'Pertx-ray diffractometer using Cu Kα radiation (λ=0.15418 nm, 45 kV, 40 mA,0.05° step, 4.0 s/step). We also examined the solid remnants of powdersthat had been washed in deionized water several times to dissolve theCDP and leave the deposited metal particles intact, obtainingdiffraction patterns absent of the strong CDP peaks. Rietveld refinementwas performed on these patterns using the Philips X'Pert Plus software.[Philips Analytical B.V., Philips X'Pert Plus, 1999]

Preparation of CDP (Powder)

Coarse-grained CsH₂PO₄ powder was synthesized as described previously.[Boysen, D.; Uda, T.; Chisholm, C.; Haile, S. Science 2004, 303, 68]Cs₂CO₃ (Alfa Aesar, 99.9%) and H₃PO₄ (ACS, 85% w/w aqueous solution)were combined in a molar ratio of 1:2 in aqueous solution andsubsequently precipitated in methanol, followed by drying of thecollected solid in air at 120° C. The coarse powder was tumbled in alow-energy ball mill with 2 mm diameter ZrO₂ spherical milling media inmethanol for 15 hours. The milled powder was recovered by filtrationthrough a 270-mesh stainless steel screen and washing of the millingmedia with copious methanol. After allowing the slurry to settle for 24hours, the supernatant methanol was poured off and replaced withtoluene. This slurry was also allowed to settle and the toluene wasremoved by boiling at 120° C. overnight. The fine CDP powder thatresulted from this treatment had an average particle size of ˜1 μm and aBrunauer-Emmett-Teller (BET) specific surface area of 2.4 m²/g measuredby nitrogen adsorption with a Micromeritics Gemini VI 2390 surface areaanalyzer.

Example 1. Platinum on CDP (Powder)

A borosilicate glass vial was filled with 80 mg of platinum(II)(2,4)pentanedionate (also known as Platinum acetylacetonate,Pt(acac)₂ and 200 mg of CsH₂PO₄ (CDP) powder. The sample vial was cappedand shaken vigorously by hand until the powder mixture appeared to be auniform, pale yellow color. The cap of the vial was removed and the vialtransferred to a vacuum oven. A separate 3.7-mL vial was then filledwith 2.1 mL of deionized water and placed in the oven far from the vialcontaining the powder mixture. The oven was evacuated to 0.17 bar with arotary vane vacuum pump and purged with dry N₂ three times. The oven wasthen evacuated again to 0.30 bar, and all valves were sealed. Thethermostat was set to 210° C. and heating was started immediately,reaching equilibrium after approximately 75 min at a temperature of 210°C. and a total pressure of 0.8 bar. After 15 h at 210° C., the heaterwas shut off, the water vapor was evacuated, and the oven was allowed toreach room temperature. This treatment deposited 16.1 wt % Ptnanoparticles on the surface of the CDP substrate.

Example 2. Platinum on CDP (Porous Disc)

100 mg of Pt(acac)₂ was applied in powder form to the top surface of aporous disc composed of 500 mg CsH₂PO₄ 0.75 inches in diameter, formedby compression of CsH₂PO₄ powder at 8 MPa. Metal deposition on CsH₂PO₄proceeded at 210° C. in a N2/water vapor atmosphere via a protocolidentical to the procedure in Example 1. This treatment deposited acontinuous, electrically conductive film of Pt nanoparticles on thesurface and within the pore structure of the CsH₂PO₄ substrate. A totalof 22 mg of Pt was deposited. Scanning electron micrograph of Ptnanoparticles after processing is shown in FIG. 4.

Example 3. Platinum/Palladium Alloy on CDP (Powder)

A glass shell vials was filled with 300 mg of CsH₂PO₄, 47.5 mg Pt(acac)₂and 147.5 mg Pd(acac)₂. Gentle shaking was used to mix the CsH₂PO₄ andPt and Pd precursors in the vial. Metal deposition on CsH₂PO₄ proceededat 210° C. in a N2/water vapor atmosphere via a protocol identical tothe procedure in Example 1. This treatment deposited 20 wt % Pt₂₀Pd₈₀alloy nanoparticles on the surface of the CsH₂PO₄ substrate.

FIG. 5 shows scanning electron micrographs of the Pt—Pd alloy depositedon CsH₂PO₄ via method similar to that described in this example. Pt 4fand Pd3d XPS spectra for Pd₈₀Pt₂₀ nanoparticles formed on the surface ofCsH₂PO₄ particles after processing similar to that described in thisexample are shown in FIG. 6. Samples were washed in water to removeCsH₂PO₄ particles and leave only Pt—Pd nanoparticles. Scanning electronmicrograph (top) and associated EDS spectrum (bottom) of Pt—Pd alloydeposited on CsH₂PO₄ are shown in FIG. 7.

Example 4. Platinum on Alumina

A powder of 30 mg of platinum(II)(2,4)-pentanedionate (Pt(acac)₂) wasapplied to the surface of an anodic alumina (Al₂O₃) membrane (9 mg, 13mm diameter, 200 nm pore size). The powder-covered disc was transferredto a vacuum oven containing 2.3 mL of DI water in a glass vial. Metaldeposition within the porous alumina structure proceeded at 210° C. in aN₂/water vapor atmosphere via a protocol identical to the procedure inExample 1. This treatment deposited 12 mg of Pt nanoparticles (57 wt %)within the porous aluminum oxide.

Example 5. Ruthenium Oxide on Carbon Black

A powder of 100 mg of ruthenium (III)(2,4)-pentanedionate (Ru(acac)₃)was added to 100 mg carbon black (Cabot Vulcan XC-72R) in a 4.5 mLcapacity glass vial. Gentle shaking was used to mix the carbon andRu(acac)₃ precursor in the vial. Metal deposition on CDP proceeded at270° C. in a N2/water vapor atmosphere via a protocol otherwiseidentical to the procedure in Example 1. This treatment deposited 20 wt% amorphous ruthenium oxide nanoparticles on the surface of the carbonsubstrate.

Example 6. Platinum on CDP (Powder) with Dichloromethane

A borosilicate glass vial was filled with 80 mg of platinum(II)(2,4)pentanedionate (also known as Platinum acetylacetonate,Pt(acac)₂, 200 mg of CsH₂PO₄ (CDP) powder, and 2 ml of dichloromethane(DCM). The sample vial was capped and shaken vigorously by hand untilthe Pt(acac)₂ dissolved, giving the slurry mixture a uniform yellowcolor. The cap of the vial was removed and the DCM allowed to evolve.The vial, with dried powder of mixed Pt(acac)₂ and CDP, was thentransferred to a vacuum oven. A separate 3.7-mL vial was then filledwith 2.1 mL of deionized water and placed in the oven far from the vialcontaining the powder mixture. The oven was evacuated to 0.17 bar with arotary vane vacuum pump and purged with dry N₂ three times. The oven wasthen evacuated again to 0.30 bar, and all valves were sealed. Thethermostat was set to 210° C. and heating was started immediately,reaching equilibrium after approximately 75 min at a temperature of 210°C. and a total pressure of 0.8 bar. After 15 h, the heater was shut off,the water vapor was evacuated, and the oven was allowed to reach roomtemperature. This treatment deposited 16.1 wt % Pt nanoparticles on thesurface of the CDP substrate.

Example 7. Platinum on CDP (Porous Disc) with Chloroform

100 mg of Pt(acac)₂ dissolved in 1 ml of chloroform (CLF), was asapplied to the top surface of a porous disc composed of 500 mg CsH₂PO₄0.75 inches in diameter, formed by compression of CsH₂PO₄ powder at 8MPa. The solution was absorbed into the porous disc and dispersedevenly, resulting in a uniform yellow color for the disc. The CLFevolved rapidly, to leave a uniform coating of Pt(acac)₂ on the CDPparticles of the porous disc. Metal deposition on CsH₂PO₄ proceeded at210° C. in a N2/water vapor atmosphere via a protocol identical to theprocedure in Example 1. This treatment deposited a continuous,electrically conductive film of Pt nanoparticles on the surface andwithin the pore structure of the CsH₂PO₄ substrate. A total of 22 mg ofPt was deposited.

Example 8. Platinum/Palladium Alloy on CDP (Powder) with Dichloromethane

A glass shell vials was filled with 300 mg of CsH₂PO₄, 47.5 mg Pt(acac)₂and 147.5 mg Pd(acac)₂ and 3 ml of dichloromethane (DCM). The samplevial was capped and shaken vigorously by hand until the Pt(acac)₂ andPd(acac)₂ dissolved, giving the slurry mixture a uniform yellow/orangecolor. The cap of the vial was removed and the DCM allowed to evolve.The vial, with dried powder of mixed Pt(acac)₂ and CDP, was thentransferred to a vacuum oven. Metal deposition on CsH₂PO₄ proceeded at210° C. in a N2/water vapor atmosphere via a protocol identical to theprocedure in Example 1. This treatment deposited 20 wt % Pt₂₀Pd₈₀ alloynanoparticles on the surface of the CsH₂PO₄ substrate.

Example 9. Platinum/Palladium Alloy on Copper

A glass vial was filled with 10 mg Pt(acac)₂ and 30 mg Pd(acac)₂. Gentleshaking was used to mix the Pt and Pd precursors in the vial. The mixedprecursors were spread over a high surface area copper TEM grid so thatit was covered in powder. Metal deposition on the copper grid proceededat 210° C. in a N2/water vapor atmosphere via a protocol identical tothe procedure in Example 1. This treatment deposited alloy nanoparticleson the surface of the CsH₂PO₄ substrate of approximate compositionPt₂₀Pd₈₀. FIG. 8 shows scanning transmission electron micrograph Pd—Ptnanoparticles formed on a copper TEM grid (inset) via a proceduresimilar to that described in this example. The single-particle EDSspectrum shows the presence of Pt and Pd, indicating alloy formation.

Example 10. Platinum on Silicon Carbide

A powder of 100 mg of Pt(acac)₂ was added to 200 mg nanoscale siliconcarbide (MKnano, Product number: MKN-SiCb-040) in a 4.5 mL capacityglass vial. Gentle shaking was used to mix the Pt(acac)₂ precursor andsilicon carbide in the vial. Metal deposition on the silicon carbideproceeded at 210° C. in a N2/water vapor atmosphere via a protocolotherwise identical to the procedure in Example 1. This treatmentdeposited 20 wt % platinum nanoparticles on the surface of the siliconcarbide substrate. FIG. 9 shows Cu Kα x-ray diffraction patterns of Ptnanoparticles deposited on nanoscale silicon carbide via a methodsimilar to that described in this example. The inset image shows ascanning electron micrograph of the sample.

Example 11. Platinum on Carbon (Powder)

A powder of 100 mg of Pt(acac)₂ was added to 200 mg carbon (Cabot VulcanXC-72R) in a 4.5 mL capacity glass vial. Gentle shaking was used to mixthe Pt(acac)₂ precursor and carbon powder in the vial. Metal depositionon the carbon proceeded at 210° C. in a N2/water vapor atmosphere via aprotocol otherwise identical to the procedure in Example 1. Thistreatment deposited 20 wt % platinum nanoparticles on the surface of thecarbon substrate.

Example 12. Platinum on CDP (Powder)

Experimental Section

Four 3.7 mL borosilicate glass scintillation vials were filled with 20mg, 40 mg, 80 mg and 160 mg of Pt(acac)₂ (Alfa Aesar, Pt 48% min,product No. 10526), respectively. To each of these vials was added 200mg of fine CsH₂PO₄ (CDP) powder. The sample vials were capped and shakenvigorously by hand until the color of the powder mixture appeared auniform, pale yellow. The caps of the vials were then removed and thevials transferred to a vacuum oven (VWR 1400E). A separate 3.7 mL vialwas then filled with 2.1 mL of deionized (DI) water and placed in theoven far from the vials containing the powders. The oven was evacuatedwith a rotary vane vacuum pump and purged with dry N₂ three times. Theoven was then evacuated again to 0.30 bar and all valves were sealed.The thermostat was set to 210° C. and heating was started immediately.The temperature and pressure of the oven were monitored with abimetallic strip and a Bourdon tube, respectively. The chamber reachedequilibrium after approximately 75 minutes at 210° C. and a totalpressure of 0.8 bar.

After 15 hours at 210° C., the heater was shut off, the water vapor wasevacuated, and the oven was allowed to reach room temperature. Thesample powders were removed from the vials by gentle shaking onto waxedweighing paper. The powders were weighed and then passed through astainless steel screen with 53 μm wire separation to homogenize theparticulates.

Results and Discussion

A scanning electron micrograph of a CDP-Pt(acac)₂ mixture (29 wt %Pt(acac)₂) is shown in FIG. 15. The large prismatic Pt(acac)₂crystallites are easily distinguishable from the semi-sphericalagglomerates of CDP.

After thermal treatment, the powders were weighed and each was found tohave lost mass equal to the volatile weight fraction of the Pt(acac)2component (52% of the precursor mass). Identically prepared mixturespowders were treated a second time at 210° C. in vacuum to check foradditional mass loss due to unreacted precursor; none was measured.

High magnification SEM imaging of the powders after thermal treatmentshows no remaining precursor particles and reveals the surface of theCDP particles to be conformally covered with a thin layer ofnanoparticles. As the amount of Pt(acac)₂ precursor is increased, thethickness of the nanoparticle layer on the CDP qualitatively increases.The change of the surface coverage and morphology of the powders withincreasing precursor weight fraction is shown in FIG. 10.

The chemical composition of each powder as measured by EDS comprised theelemental components of CsH₂PO₄ and Pt. Though the strong characteristicP K peak and the Pt M peak nearly overlap at approximately 2 kV, weattempted to quantify Pt content by means of the Pt/P weight ratio. ThePt/Cs ratio, though not confounded by overlapping peaks, was expected tolack accuracy due to a missing energy calibration standard for Cs on theinstrument used. Indeed, the measured ratio of Cs to P deviates byapproximately 10% to 20% from the true value of 1:1. Nevertheless, theresults in Table 1 show very good agreement for the total amount of Ptcompared to the calculated Pt/P weight ratios based on precursor Ptcontent. The sampling depth of the EDS technique is approximately 1 μm,similar to the primary CDP particle size, and it appears that entireparticulate volumes have been sampled.

TABLE 1 EDS Measurements of Pt/P and Cs/P Weight Fractions in Pt:CDPPowders. Pt/P Pt/P deviation Cs/P error Pt wt % Pt/P (meas.)[x_(at)]^(a) (calc.) [%] [%] 5 0.345 ± 0.052 [0.06] 0.361 −4.4% 19.6% 90.604 ± 0.053 [0.10] 0.722 −16.3% 11.0% 17 1.450 ± 0.074 [0.24] 1.4440.4% 11.1% 29 2.494 ± 0.102 [0.41] 2.888 −13.6% 10.5% ^(a)Measured Pt/PAtomic Ratio is Shown in brackets.

High-resolution XPS scans of the Pt 4d doublet for each sample arepresented in FIG. 16. The Pt 4d photoelectron peak was chosen due to thenear overlap of the Cs 4d with the more intense Pt 4 f doublet. Threepeak components were required to adequately fit the data, and aredenoted as peak 1 (solid line) and peak 2 (dashed line), and peak 3 (notshown). Peak 3 is a small C 1s loss peak at 313 eV and is not shown forthe sake of clarity. Peaks 1 and 2 were fit as a doublet with a fixedspin-orbit splitting of 16.9 eV. [Zsoldos, Z.; Hoffer, T.; Guczi, L. J.Phys. Chem. 1991, 95, 798-801] All peak centers are given in terms ofthe center of the 4d_(5/2) portion of the doublet. Peak 1, at314.97±0.10 eV, is assigned to Pt metal in accordance with tabulateddata [National Institute of Standards and Technology, NIST X-rayPhotoelectron Spectroscopy Database, Version 3.5, 2003. See web addresssrdata.nist.gov/xps/], though the accepted position for the peak is314.60 eV. Peak 2, at 318.88±0.19 eV, is assigned to PtO₂, though PtOand Pt(OH)_(x) species cannot be ruled out due to the width of thepeaks, which show similar chemical shifts when observed in the Pt 4 fpeak. [Shukla, A.; Neergat, M.; Bera, P.; Jayaram, V.; M S, J.Electroanal. Chem. 2001, 504, 111-119; Hamnett, A.; Weeks, S. A.Electrochim. Acta 1987, 32, 1233-1238; Allen, G.; Tucker, P.; Capon, A.;Parsons, R. J. Electroanal. Chem. 1974, 50, 335-343; Blackstock, J.;Stewart, D.; Li, Z. Appl. Phys. A 2005, 80, 1343-1353.] The positions ofthe peaks and the ratio of Pt metal to oxide species are listed in Table2. Pt oxides are less prevalent at higher Pt loadings. This is mostlikely due to Pt nuclei already present on the CDP surface acting tocatalyze the Pt(acac)₂ decomposition reaction, which we discuss in moredetail below.

TABLE 2 XPS Peak Positions and Ratio of Pt⁰ to Pt oxides for Pt:CDPpowders. Pt 4d_(5/2) [eV] Pt [wt %] peak 1 peak 2 Pt/Pt_(oxide) 5 314.87318.85 6.4 9 314.97 318.81 7.7 17 314.92 318.66 9.1 29 315.1 319.10 9.7

The interpretation of O 1s photoelectron peaks in the Pt:CDP system isquite complicated. High-resolution scans from the sample powders arepresented in FIG. 17. Three peak components fit the spectra quite well,but the assignment of these peaks is difficult. The largest of thecomponents, at 532.75±0.24 eV, is likely a from nCH₂CH₂O-type speciesresulting from incomplete ligand decomposition. The peak at 530.76±0.11eV decreases monotonically with Pt loading, and we can assign it withsome confidence to the PO₄ group of the underlying CDP. The peak at531.815±0.19 eV is likely to contain components from surface hydroxylgroups, PtO₂, and Pt(OH)_(x). We compared the relative magnitudes of thepeaks by normalizing them to the intensity of the most intense peak at532.75 eV. These results are summarized in Table 3.

C 1s peak scans are presented in FIG. 18. In addition to theadventitious peak that has been used as an internal energy reference at284.8 eV, the C 1s peaks also show evidence of a chemical shift at287.24±0.14 eV. This peak is assigned most generally to organicfragments from incomplete decomposition of the Pt(acac)₂ precursor. Theratio of adventitious carbon to unknown carbon species is presented inTable 4. There may be evidence for a small autocatalytic effect fordecomposition of organic contaminants, but there is no correlation ofthat effect beyond the lowest Pt loading.

TABLE 3 XPS Peak Positions and Intensities for O 1s Peaks for Pt:CDPPowders. Peak 1 pos. Peak 2 pos. Peak 2 Peak 3 Peak 3 Pt wt % [eV] [eV]int. pos. int. 5 532.9 531.6 0.51 530.8 0.25 9 532.9 531.8 0.49 530.80.25 17 532.8 531.9 0.55 530.8 0.24 29 532.4 532.0 0.51 530.6 0.21Assignment nCH₂CH₂O OH, PtO₂, PO₄ Pt(OH)_(x)

TABLE 4 XPS Peak Positions and Ratio of Adventitious C to Unknown C forPt:CDP Powders. Pt [wt %] C [eV] C_(unknown) [eV] C/C_(unknown) 5 284.8287.26 2.82 9 284.8 287.35 1.80 17 284.8 287.03 1.89 29 284.8 287.302.14

High resolution scans of the P 2p peaks for the samples are shown inFIG. 19. Only one peak component is present at 133.8±0.08 eV, which isexpected given the nature of the ionic bonding in the CDP support. Theintensity of the peak decreases with increasing platinum content, due tocombined effect of increasing Pt coverage and the absorption of theemitted P photoelectrons by the Pt nanoparticle overlayer.

To estimate the thickening of the Pt nanoparticle layer on the CDPsurface, the Pt/P atomic ratio was calculated using the integratedintensities of the Pt 4d5/2 and P 2p peaks and the associated atomicsensitivity factors. This approach does not explicitly account for theattenuation of the P 2p photoelectrons by the supported Pt nanoparticlelayer or the varying coverage of the support at low Pt loadings. Thecalculated Pt/P atomic ratios for 5, 9, 17, and 29 bulk wt % Pt are1.76, 3.16, 4.25, and 6.27, respectively. These results aresignificantly increased with respect to the EDS results in Table 1 dueto the extreme surface sensitivity of the XPS technique.

The XPS data show that Pt⁰ is the dominant species present in thesupported nanoparticles, but that Pt^(II) and Pt^(IV) are also likely tobe present. Assorted organic species differentiable from adsorbed waterand hydrocarbons are also present, though the precise makeup of thesecompounds cannot be inferred unambiguously. We note that Pt oxides arenot deleterious for fuel cell performance, because the oxides arereduced electrochemically during operation. High concentrations oforganic species may block catalytic sites, however.

To visually estimate the maximum average thickness of the Ptnanoparticle films, the assumptions of 100% Pt yield and perfectconformal coating were adopted. Based on these assumptions, thethickness of the Pt film is

$t = \frac{1000Ø}{\rho_{Pt}\sigma_{cdp}}$Here, t is the film thickness in nm, φ is the ratio of platinum to CDPmass, ρ_(Pt) is the density of Pt (taken to be 21.46 g/cm³) and σ_(cdp)is the specific surface area of the CDP in m²/g. Nitrogen adsorptionmeasurements found the specific surface area of the CDP particlesdiscussed here to be 2.4 m2/g. The maximum uniform film thicknessesexpected from the experimental samples (10 mg, 20 mg, 40 mg and 80 mg ofPt per 200 mg of CDP) are thus 1 nm, 2 nm, 4 nm, and 8 nm, respectively.From inspection of the SEM micrographs this appears to be a useful modelfor characterizing the results of the Pt deposition process. FIG. 20shows a powder sample prepared with a 50 wt % Pt loading, and is themost illustrative. A cohesive, thick Pt film appears edge-on, andpixel-based measurement of the film thickness with the program ImageJ[Rasband, W. S. ImageJ. See web address rsb.info.nih.gov/ij/] returned athickness of 42±3 nm, while the model suggests a film thickness of 53nm.

TABLE 5 Pt Structural Parameters from XRD Loading [wt % Pt] LatticeParameter [nm] Particle Size [nm] 5 0.3916 2.4 9 0.3912 2.8 17 0.39043.2 29 0.3910 3.7

X-ray diffraction patterns from the as-coated powders and the washedpowders are shown in FIG. 11. In the upper portion of the figure, thesharp, highly multiple diffraction peaks of CDP are overlaid on anenvelope of broad peaks with the fcc symmetry of Pt. The intensity ofthe Pt peaks increase with increasing Pt content. In the lower portionof the figure, clear Pt fcc patterns are evident for the washed samples.Rietveld refinement of the washed Pt patterns using the fcc Pt structure[Swanson, H.; Tatge, E. National Bureau of Standards (U.S.), Circular1953, 539, 1] returned for each sample a lattice parameter and width ofthe Pt(111) peak (d-spacing=0.2265 nm; 2θ=39.765°. Average Pt particlediameters t were calculated using the Scherrer equation:

$t = \frac{0.9\;\lambda}{B\;\cos\;\theta}$Here, B is the full width at half maximum intensity of the peak, θ isthe Bragg angle, and for Cu Kα₁, λ=0.154060 nm. The results arepresented in Table 5 below.

The refined lattice parameters are systematically lower than the valuefor bulk platinum of 0.39231 nm. Lattice contraction in small metalnanoparticles is a well-known phenomenon and has been observed for Pt[Klimenkov, M.; Nepijko, S.; Kuhlenbeck, H.; Bäumer, M.; R, Surf. Sci.1997, 391, 27-36], Pd [Lamber, R.; Wetjen, S.; Jaeger, N. Phys. Rev. B1995, 51, 10968-10971], and other fcc metals. [Montano, P.; Shenoy, G.;Alp, E.; Schulze, W.; Urban, J. Phys. Rev. Lett. 1986, 56, 2076-2079]The increase of the average Pt grain size with increased precursorloading is indicative of a nucleation and growth process in which Ptnuclei initially formed on the surface of CDP act to catalyze further Ptreduction and particle growth.

The polarization curves drawn from each of the experimental electrodes,after correction for ohmic resistance, are shown in the upper portion ofFIG. 21. The large overpotentials are typical of SAFC cathodes [Haile,S.; Chisholm, C.; Sasaki, K.; Boysen, D.; Uda, T. Faraday Discuss. 2007,134, 17; Chisholm, C.; Boysen, D.; Papandrew, A. B.; Zecevic, S.; Cha,S.; Sasaki, K. A.; Varga, A; Giapis, K. P.; Haile, S. M. Electrochem.Soc. Interface 2009, 18, 53-59], which have a paucity of active catalystsites. Still, the electrode with 0.88 mg/cm² of Pt has a far higherovervoltage than the electrodes with thicker Pt films. We quantifiedthese overpotentials by measuring the difference between the cellvoltage and 1.1 V at a current density of 50 mA/cm². The referencevoltage was arbitrarily chosen to be approximately equal to thetheoretical Nernst potential for H₂/air in the presence of 0.3 atm H₂Oat 250° C., which is 1.12 V for a hydrogen reversible electrode underthese conditions. The measured overpotentials and ohmic resistances areplotted against Pt loading in the lower portion of FIG. 21.

The large increases in both values at 0.88 mg/cm² are related. At thisloading the average particle size from XRD is 2.4 nm (Table 5) while themaximum calculated average film thickness is 1 nm. If we discount apreponderance of anisotropic particle shapes it is clear that completecoverage of the available CDP surface area by Pt is not possible. As thePt coverage of the CDP network becomes sparse, the electronic pathdensity drops, leading to an attendant increase in the ohmic resistance.Similarly, Pt nanoparticles that are not connected to the electronicconduction network cannot be active catalytically, and the overpotentialfor the electrode is increased.

Above this threshold for complete coverage of the CDP network by Pt,there is no effect of the addition of additional Pt, as shown by thenearly identical activation overvoltages of the remaining samples (FIG.21). The increase of the Pt particle diameter from 2.8 nm to 3.7 nm withincreased loading has no impact on the oxygen reduction activity of theelectrodes, despite the strong dependence of oxygen reduction activityon Pt particle diameter [Mukerjee, S.; McBreen, J. J. Electroanal. Chem.1998, 448, 163-171; Mayrhofer, K. J. J.; Blizanac, B. B.; Arenz, M.;Stamenkovic, V. R.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2005,109, 14433-40]. This is clear evidence that it is the electrochemicalsurface area of the CDP electrolyte, not the Pt catalyst, that dominatesthe performance of SAFCs. In the case of the highest loading tested,fuel cell performance can be seen to decrease slightly, but at the highcurrent densities indicative of transport losses, rather than kineticlosses. We attribute this effect to an inhibition of proton transportrelated to constricted particle-to-particle proton conduction pathwaysin the presence of increasingly thick Pt nanoparticle layers.

Comparisons of the Pt:CDP electrode to the first generation standardSAFC electrodes are favorable. The standard electrode consists of amechanical mixture of 37.5% Pt black (HiSPEC 1000, Alfa Aesar), 37.5%CDP, 12.5% 0.4Pt on XC-72R (HiSPEC 4000, Alfa Aesar), and 12.5%naphthalene as a fugitive binder/poreformer. The electrode weighed 50mg, and had an area specific platinum loading of 7.5 mg/cm². It wasformed in the same fashion as the Pt:CDP electrodes, but bonding of theelectrode to the membrane proceeded at 120 MPa for 1 minute. Arepresentative IR-free polarization curve from the standard electrode iscompared to an experimental electrode in FIG. 22. The experimentalelectrode exhibits an overvoltage at 50 mA/cm² that is lower by 50 mVand has a voltage in excess of the standard electrode at the highercurrent densities. Gas diffusion losses are negligible in the Pt:CDPelectrode due to its open structure as a result of being laminated at 8MPa. The standard electrode is far more dense and thus more acutelyaffected by gas diffusion losses. A comparison of the two electrodemicrostructures is shown in FIG. 23.

The experimental Pt:CDP electrodes display adequate stability overmoderate operation time. An electrode with 1.75 mg/cm² of platinum and50 mg of CDP was operated at a constant current density of 200 mA/cm² at250° C., with 30/60 sccm H₂/air at 75° C. dew point. Over the course of200 hours, the experimental electrode decreased in voltage at a linearrate of 110 μV/hr. Under the same operational conditions, the standardelectrode degrades at the same rate. Voltage decay in SAFCs is a keyimpediment to commercial adoption, and is still not completelyunderstood. We attribute the major modes of voltage loss to changes inthe cathode microstructure related to the properties of the CDPelectrolyte. CDP is superplastic in its superprotonic phase, leading tolarge enhancements of the sintering rate of the individual particles inthe electrode during fuel cell operation. Sintering closes activecatalyst sites to reactant gases, reducing overall catalytic activityand impairing gas diffusion. A concurrent mechanism that has beenobserved in post-operation SEM studies of Pt:CDP electrodes is areorganization of the platinum nanoparticles on the surface of thesupporting CDP, shown in FIG. 24. The high degree of dynamic disorder inthe CDP lattice in the superprotonic phase may foster fast diffusion ofPt atoms, leading to clustering of Pt and eventual disruption of thecatalytic-electronic network on the nanoscale. Alternately, Pt may betransported by a vehicle mechanism due to the formation of small domainsof the liquid-like metastable phase of CDP.

We are not aware of other methods of platinum deposition that result inthe continuous nanoparticle networks that enable the demonstratedelectrode advances. The nature of these networks suggest that thesurface chemistry of CDP has considerable influence on the depositionprocess. An infrared spectroscopy study of Pt(acac)₂ interactions withsurface sites in alumina supports a dissociative mechanism facilitatedby surface acidity, and CDP is considerably more acidic than alumina.

In that case, following physisorption of Pt(acac)₂ on CDP from the vaporphase, chemisorption will proceed as one acac ligand is dissociated byligand exchange with surface hydroxyls. The surface of CDP will thencontain protonated acac, acacH, and a Pt—O bond. This reaction isthermodynamically favorable and will yield an ionic Pt—O intermediate.The loss of the second acac ligand will then proceed rapidly, yielding aPt^(II) oxo-type species. Reduction to the dominant Pt⁰ species islikely to occur via a two-electron process concomitant with theoxidation of acacH. The minority oxidized Pt species detected by XPS mayresult from an incomplete reaction or from reaction of Pt⁰ withactivated surface-bound water. Based on thermogravimetric analysis-massspectroscopy (TGA-MS) experiments of the decomposition of Pt(acac)₂ onrelated surfaces, acetone, CO, and CH₄ are probable byproducts of ligandoxidation. CO may result from decarbonylation of a biacetylintermediate, as observed in other ligand-mediated reductions of metalcenters. It is also possible that radical mechanisms may be occurringunder the reaction conditions.

While we cannot rule out an associative mechanism for CDP, in which aphosphate group acts as a nucleophile, followed by the loss of anionicacac, this is not as likely, given the favorable hydrogen-bondinginteraction that may exist between the phosphate group and acac ligandand the rather poor nucleophilicity of phosphate (as opposed tophosphines).

Beyond its primary role in preventing the dehydration phasetransformation in CDP during the deposition process, water may haveancillary effects on Pt deposition. First, water vapor may decrease theactivation energy required for deposition, as was observed in relatedsurface-mediated processes. The transition state will involve astructure that has a formation of a Pt^(II)-surface bond and thattransition state may be stabilized by hydrogen bonding with water. Thepresence of water in the chamber may also assist the protonation of theacac ligand to form neutral acetylacetonate, which will be adsorbed onCDP. As CDP has acidic sites, this may only be a secondaryconsideration. Water may also occupy surface sites to restrict diffusionof Pt atoms, giving a higher density of nucleation and a smaller averagecrystallite size.

The hypothesized role of the substrate surface suggests much broadercatalysis applications of this technique. Carbon, silica, and aluminaare workhorse catalyst supports with acidic surfaces and copiousadsorbed water. Success in applying our technique to these supportsseems likely and offers single-stage processing, fine control over themetal loading, and very high precursor yield. Moreover, the continuousPt films formed by the addition of larger amounts of precursor suggestthe facile synthesis of novel core-shell nanoparticle structures. Thedemonstrated coating of the available electrolyte surface area hasprofound implications for further development of SAFC technologies,using the reported technique. To increase the cathode activity anddecrease the significant oxygen reduction overpotential, a higherdensity of active triple points must be created. The most direct way toachieve this aim is to decrease the average particle size of the CDP inthe electrode. As a consequence, the surface area of the electrolyteparticles must increase.

Of course, following our development above, this would necessitate acorresponding increase in platinum loading for adequate surface coverageand catalytic activity. However, for an electrolyte with a relativelylow proton conductivity, such as CDP, the reaction zone for oxygenreduction is highly biased toward the interface with the membrane. It isthus possible that platinum loading may be dropped significantly by theincorporation of a substantially thinner, highly nanostructuredelectrode.

Example 12. Preparation of Electrodes

Electrodes were formed by spreading the coated powders over ananode-supported half-cell with a total area of 2.85 cm² and a CDPmembrane thickness of 50 μm, similar to those described elsewhere [Uda,T.; Haile, S. Electrochem. Solid-State Lett. 2005, 8, A245], followed bycompression of the powder at 8 MPa for 3 seconds. The mass of CDPelectrolyte in each electrode was kept constant at 50 mg, and thearea-specific platinum loading of the electrodes was 0.88 mg/cm², 1.75mg/cm², 3.5 mg/cm², and 7 mg/cm², respectively. Nickel foam currentcollectors (INCO) were sealed against each electrode with PTFE tape(McMaster-Carr, part #4591K11) to complete the fabrication of the SAFCMEA.

Each MEA was loaded into a stainless steel test fixture and installed ina test rig. The test fixture was heated to 250° C. and supplied with ananode reactant flow of 30 sccm of H₂ and a cathode flow of 60 scmm ofair. Both reactant streams were hydrated to a dew point of 75° C. After12 minutes of equilibration, a polarization curve was measured with agalvanostat (Keithley 2420) from zero current in increasing steps at arate of 0.5 s per current step. Up to 200 mA, 200 current steps weredistributed according to the functioni[mA]=10^(x/2.3)−1,where the value of x ranges from 0 to 199 in unit steps; thereafter thecurrent step was 5 mA. The maximum current for the measurement was thecurrent at which the cell voltage was zero. Immediately after thepolarization measurement, the ohmic resistance of the cell was measuredby the current interrupt technique. [Chisholm, C. R. I.; Boysen, D. A.;Hettermann, M. L.; Papandrew, A. B. U.S. Pat. No. 7,577,536, 2009.]

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, one of skill in the art will appreciate that certainchanges and modifications can be practiced within the scope of theappended claims. In addition, each reference provided herein isincorporated by reference in its entirety to the same extent as if eachreference was individually incorporated by reference.

What is claimed is:
 1. A method of preparing metal particles or metaloxide particles on a substrate, comprising: heating a chamber to asingle temperature, wherein the chamber contains a reaction mixturecomprising a metal precursor or metal oxide precursor and a substrate,such that the metal precursor or metal oxide precursor is vaporized inthe chamber and metal particles or metal oxide particles are formed onthe substrate.
 2. The method of claim 1, wherein the metal precursor ormetal oxide precursor comprises at least one metal selected from thegroup consisting of a transition metal and a post-transition metal. 3.The method of claim 1, wherein the metal precursor or metal oxideprecursor comprises at least one metal selected from the groupconsisting of Pt, Pd, Ir, Ru, Ni, Co, Fe, Cu, V, Cr, Ti, Ta, Mn, Mo, Nb,and Sn.
 4. The method of claim 1, wherein the metal precursor or metaloxide precursor comprises at least one metal selected from the groupconsisting of Pt, Pd and Ru.
 5. The method of claim 1, wherein the metalprecursor or metal oxide precursor comprises Pt.
 6. The method of claim1, wherein the metal precursor or metal oxide precursor comprises Pt andPd.
 7. The method of claim 1, wherein the substrate comprises at leastone member selected from the group consisting of a metal, a carbonmaterial, a metal oxide, a silica material, a polymeric material, asolid acid, a solid oxide, and a metal salt.
 8. The method of claim 1,wherein the substrate comprises a solid acid.
 9. The method of claim 1,wherein the substrate comprises at least one member selected from thegroup consisting of CsH₂PO₄ and alumina.
 10. The method of claim 1,wherein the substrate comprises CsH₂PO₄.
 11. The method of claim 1,wherein the metal precursor or metal oxide precursor comprises Pt, andthe substrate comprises CsH₂PO₄.
 12. The method of claim 1, wherein thesubstrate is selected from the group consisting of a powder, a poroussubstrate and a solid substrate.
 13. The method of claim 1, wherein thesubstrate is selected from the group consisting of a porous substrateand a solid substrate, such that the metal particles or metal oxideparticles form a continuous network of particles on the surface of thesubstrate.
 14. The method of claim 1, wherein the reaction mixturefurther comprises a solvent selected from the group consisting ofdichloromethane, chloroform and carbon tetrachloride.
 15. The method ofclaim 1, wherein the heating is at a temperature of from about 20 toabout 500° C.
 16. The method of claim 1, wherein the heating is at atemperature of from about 20 to about 300° C.
 17. The method of claim 1,wherein the heating is at a temperature of from about 20 to about 210°C.
 18. The method of claim 1, wherein the heating is performed at apressure of from about 0.01 to less than 1 atm.
 19. The method of claim1, wherein the heating is performed at a pressure of from about 0.2 toabout 0.9 atm.
 20. The method of claim 1, wherein the metal precursor ormetal oxide precursor is in direct physical contact with the substrate.